Recently I was required to figure out why the CSS and HTML files were not working after the client uploaded them to the server. He provided an edited HTML file as well as the current path to CSS.
Task: Compare the working original HTML file with the client’s edited HTML file.
Solution: Use a program that compares and highlights the differences between two files.
There are plenty of Freeware and Shareware programs that allow you to compare two documents. For instance TextDiff by Angus Johnson has customisable colour highlighting to illustrate the file difference, search and replace text, multiple undo/redo of text changes and generates a report of the file difference.
Something that might take hours to troubleshoot can be done in a jiff.
As it turned out it the HTML document type was not declared correctly.
Also recommended: A fresh pair of eyes.